Nickel base cathode emissive alloy



iii tates NICKEL BASE CATHODE EMISSIVE ALLOY Donald R. Kerstetter, Emporium, Pa., assignor to Sylvania Electric Products Inc., a corporation of Massachusetts No Drawing. Application March 15, N57 Serial No. 646,193

3 Claims. (Cl. 75-171) This invention relates to the composition of a cathode base such as is utilized in electron tubes. Where such base is used, the same may be in filamentary form with an emissive coating thereon and to be directly heated, or in some other form, as a sleeve with an emissive coating thereon to be indirectly heated. In either case the base is initially coated with alkaline earth carbonates, usually a mixture of the carbonates of barium and strontium, which during tube processing are converted to the earth oxides by application of high temperatures to the base. Further prolonged heating of the oxides with moderate positive potential applied to an. adjacentelectrode renders the oxides electron emissive, with concomitant reduction of some of the oxide material to the metal. I

During tube use the emissive character of the coating is dependent on the rate of reduction of the oxide to the free metals; the greater the reduction, the greater the emission. Such reduction of the oxides takes place for various reasons. One of the reasons and with which the invention is concerned is the migration of material within the base to the interface between the base and the coating. In the past, it was found that the addition of various substances such as magnesium, silicon, and carbon to the base composition is desirable since they have a migratory characteristic and they react with the oxide coating to free its metals; cathode filamentary bases, as well as cathode sleeves, have therefore commonly been made of an alloy containing percentages of the above substances to take advantage of these properties.

It is now found that when a suitable proportion of chromium is added to the base alloy containing these substances, the chromium will not only effect an increase in emission but will bring about other unexpected and desirable results.

A proper proportion of chromium metal in a cathode base alloy for use with emissive coatings will not only release free metals from the alkali earth oxides, but will be itself oxidized, either by reduction of the alkali earth metal oxide or by reaction with residual oxygen in the tube envelope, and will deposit on the cathode base and cathode interface to form a film impervious to penetration of other metals in the alloy. This will hinder these other metals from subliming and depositing on inner portions of the tube to thereby form low resistance leakage paths between electrode elements. Also when a proper proportion of chromium is added to the cathode base alloy, the chromium will migrate at a very slow rate to the interface between base and coating, to provide a uniform and prolonged reaction between the chromium and the oxides, as for example barium oxide, so as to supply free alkali earth metal at the interface. This alkali earth metal will then migrate through the coating to the exterior face thereof supplying a greater amount of emissive material during life of the tube than where no chromium is in the base. The reaction takes place in accordance with the following formula, where barium oxide is exemplary of any alkali earth oxide:

Z,h3?,423 Patented June 3, 1958 As a result of the use of chromium in the. base alloy and the use of an emissive coating, it has been found that tube life is greatly extended since the chromium does not become eifective until after migration through the alloy base to the interface between base and emissive coating. Thus the emissive characteristics of the coating are greatly improved particularly toward the later life of the tube.

Besides the properties above, it has been found that in the proportions used, chromium will give added stiifness and rigidity to the base material.

it is an object of the invention to provide a cathode base alloy which shall impart improved life and emissive characteristics to a cathode utilizing the alloy.

It is a further object of the invention to provide a base material which shall have the effect of reducing sublimation of other metals in the alloy into the tube envelope, thereby reducing leakage in the tube during late life thereof.

It is yet another object of the invention to provide a Range, Percent Nickel The function of the aluminum in the composition is to provide reducing capability to the oxide coating on the cathode base in accordance with the following reaction:

Secondly the function of the aluminum is to form an impervious film on the cathode base to inhibit the sublimation of other elements from the cathode base and the deposition thereof on other elements of the tube with consequent changes in resistance paths or contact potentials. This film is formed not only by the reaction set forth above, but by reaction with the residual oxygen within the evacuated tube envelope. In so far as sublimation inhibiting film forming is concerned, the aluminum is an adjunct to the chromium. But it'should be understood that some chromium during prolonged tube life does migrate to the interface between metal and alkali earth oxide and does react with the oxide as described.

The function of the magnesium is to provide a good reducing potential to the oxide coating on the surface of the cathode base in accordance with the following reaction:

In addition to the reducing effects of this element, it inhibits the formation of high resistance interface barriers between the coating and the cathode base.

The function of the manganese is to increase the cold working physical characteristic of the-alloy. Also, the

element has reducing effects and inhibits the formation of high resistant interface barriers between the cathode base and the coating.

The function of the tungsten is to improve the strength of the cathode base metal.

The cobalt adds to the hot strength of the metal and improves the electrical stability of the finished tube.

The titanium in the composition acts as an activating agent at the interface and assists in the reduction of the oxides.

The remaining ingredients play minor roles in the composition or are contaminants difiicult to remove.

In a life test run on a batch of type 6AF4 tubes utilizing the 1.0% chromic alloy in comparison to a tube manufactured with a standard base alloy, not containing chromium, the following results were obtained.

Elapsed Overall Percent life, emission slump at hrs. 500 hrs.

104 Standard In a second test using another batch of the tubes the following results were found.

Tests on other types of tubes showed consistently high emission at 500 hours life of the tubes and emissions above tubes having cathodes made with the standard nonchrome alloy; the tests also showed less percentage slump in emission from initial tube life to the 500 hour mark.

The use of the above chrome alloy results in a high emissive characteristic which is maintained through the life of the tube, while the use of a non-chrome alloy results in a much lower emissive characteristic.

It has been found that where the amount of chromium is small, in the range below .2%, the desired effects are insignificant. On the other hand an increase of chromium to above 2.25% has brought about markedly inferior results in so far as attaining high emission is concerned. The best results are obtained with a chrome alloy containing between .8% and 1.3% with optimum results at 1.0%.

What is claimed as the invention is:

1. A cathode comprising an alloy base and an alkaline earth oxide coating on the base, said alloy base, by weight, containing about .04% aluminum, 1.0% chr0- rniurn, 2.0% tungsten, and 0.5% cobalt, the balance of the alloy of the cathode base being preponderately of nickel.

2. A cathode comprising an alloy base and an alkaline earth oxide coating on the base, said alloy base, by weight, containing about .04% aluminum, 1.0% chromium, 2.0% tungsten, 0.5% cobalt, .04% magnesium, 05% manganese, and .006% titanium, the balance of the alloy of the cathode base being preponderately of nickel.

3. A cathode comprising an alloy base and an alkaline earth oxide coating on the base, said alloy base, by weight, consisting of about .04% aluminum, 2.0% tungsten, 006% titanium, .5% cobalt, .0S% manganese, 006% copper, 1.0% chromium, 005% silicon, .01% carbon, .04% magnesium, and 04% iron, the balance of the alloy of the cathode base being of nickel.

References Cited in the file of this patent UNITED STATES PATENTS 1,909,916 'Duiiendack et a1. May 16, 1933 1,976,295 Randolph Oct. 9, 1934 2,194,167 Holladay Mar. 19, 1940 

1. A CATHODE COMPRISING AN ALLOY BASE AND AN ALKALINE EARTH OXIDE COATING ON THE BASE, SAID ALLOY BASE, BY WEIGHT, CONTAINING ABOUT .04% ALUMINUM, 1.0% CHROMIUM, 2.0% TUNGSTEN, AND 0.5% COBALT, THE BALANCE OF THE ALLOY OF THE CATHODE BASE BEING PREPONDERATELY OF NICKEL. 